Thin layers, deposited on a substrate and having a maximum thickness of the order of tens of micrometers, are commonly known by the term “films”, which will be used in the rest of the text.
Thin films can be produced in many ways, for example by techniques known as Chemical Vapour Deposition (CVD) or Physical Vapour Definition (PVD, such as the technique known as sputtering) and their numerous variants. Good productivity is achieved with these techniques, however fine control of the morphology and porosity of the produced film is not possible.
Nanostructured thin films have recently acquired considerable interest. These films consist of nanometric particles, i.e. of dimensions approximately between a few nanometers and a few hundred nanometers.
The interest is due to the fact that these films present functional properties which are new, or of superior quality, compared to non-nanostructured films; for example, nanostructured films are more efficient as photoluminescent sources, in the conversion of radiant energy into electrical energy (property particularly useful for the production of solar panels), or as gas sensors.
A first widely studied possible way to produce these films uses processes carried out in two steps, in which the first step is the production by various ways of powders of nanometric size and their collection, and the second step is the deposition of the powders, for example in the form of pastes or suspensions, which are deposited on the substrate by methods such as silk screen printing or the like. This approach is described in numerous patent documents. For example patent application U.S 2005/0258149 A1 describes a method in which the nanoparticles are produced in a plasma maintained at high temperature (system known in the sector as “plasma torch”) and collected directly in oil in a tank, to form a suspension which is then used to produce deposits. Patent application U.S. 2006/0159596 A1 and U.S. Pat. No. 7,052,667 B2 (the latter particularly aimed at the production of carbon nanotubes) describe methods in which the nanoparticles are produced in a high pressure plasma torch system and collected in a trap or on filters, for example of sintered metal or paper, from which they are then recovered, to be then deposited on a substrate by known methods. Finally, patent application U.S. 2009/0056628 A1 describes a method for producing nanoparticles by means of a radiofrequency plasma, and in this case too the particles are then recovered and subsequently used to produce films; alternatively, according to this document, the particles leaving the production reactor can be collected directly on a substrate, to which they adhere by electrostatic attraction (or simply by gravity).
The article “Plasma synthesis of semiconductor nanocrystals for nanoelectronics and luminescence applications”, U. Kortshagen et al., Journal of is Nanoparticle Research, (2007) vol. 9, no. 1, pages 39-52, describes the production of nano-sized powders of silicon in a non-thermal plasma. In the system of this article, a precursor of silicon (e.g., a silane) is introduced at one end of a quartz tube in which a relatively high gas pressure is kept; a plasma is generated in the whole tube, that decomposes the silicon precursor causing the creation of silicon nanoparticles; the opposite end of the tube is closed by a wall, in which an orifice is present; the wall separates the volume in the quartz tube from a chamber kept at lower pressure; the difference of pressures in the two zones causes the extraction of a jet of the mixture of gasses and silicon nanoparticles present in the quartz tube; the nanoparticles are then collected, for instance on a TEM grid. In the method of this document, as described in particular with reference to FIG. 4 therein, the silicon nanoparticles begin to form already at a position rather distant from the orifice; these particles are initially amorphous and porous, and of relatively high diameter (around 200-400 nm); as the whole volume of the quartz tube is kept in plasma conditions by means of electrodes, the initial particles, in their travel towards the orifice, continue to react and become denser (and smaller) the closer they are to the orifice; once these particles enter the lower-pressure chamber, downstream the orifice, they have already reached their final shape and dimensions (around 40-70 nm), and are collected as such.
These methods generally do not allow control of the aggregation order of the film formed from the nanoparticles, giving rise to films which are essentially disordered and often difficult to reproduce; with these films it is not possible to fully utilize the potentiality of the nanostructured films and to control them.
In this respect it has been observed that the best results in terms of functional properties of films produced from nanometric sized particles are obtained when these aggregate to form structures of greater size in accordance with a growth scheme known in the sector as “hierarchically organized”, described for example in the articles “Hierarchically organized nanostructured TiO2 for photocatalysis applications”, F. Di Fonzo et al., Nanotechnology 20 (2009) 015604, and “Hierarchical TiO2 Photoanode for Dye-Sensitized Solar Cells”, F. Sauvage et al., Nano Letters 10 (2010).
The technique used in the stated article is “pulsed laser deposition” (PLD), in which a source of the material to be deposited is caused to evaporate by a sequence of laser radiation pulses; the evaporated material aggregates to form bunches of atoms or molecules (known in the sector as clusters) which, upon deposition on a substrate, undergo self-organization, according to the process parameters, into dense compact columnar, open columnar or dendritic structures. The limit to this technique, which prevents its application on an industrial level, is its low productivity, due to the fact that the laser can irradiate surface areas of a maximum of a few square centimeters, with an evaporation rate (and consequent deposition on the substrate) of the order of some tens of ng/s or nm/s in terms of the thickness of the deposited film based on its density.
International patent application WO 2009/032654 A1 describes an alternative method for producing hierarchically organized nanostructured films which leads to results similar to those of the aforementioned article. This method consists of introducing into a reaction zone a vapour stream of a compound containing a metal, a fuel vapour stream and an oxidant vapour stream, and cause a combustion to occur within the reaction zone; the combustion results in the formation of fumes of nanoparticles of the metal oxide, which are then deposited onto a substrate maintained at a controlled temperature. This technique presents the drawbacks of being relatively unclean, requiring a fuel to be burned, hence not allowing optimal control of the chemistry of the deposited film.